Scanning laser projector system utilizing photodiodes inside scan area but outside of projection area for feedback

ABSTRACT

A scanning laser projector includes an optical module and projection engine. The optical module includes a laser generator outputting a laser beam, and a movable mirror scanning the laser beam across an exit window defined through the housing in a scanning pattern wider than the exit window such that the laser beam is directed through the exit window in a projection pattern that is smaller than and within the scanning pattern. A first light detector is positioned about a periphery of the exit window such that as the movable mirror scans the laser beam in the scan pattern, at a point in the scan pattern where the laser beam is scanned across an interior of the housing and not through the exit window, the laser beam impinges upon the first light detector. The projection engine adjusts driving of the movable mirror based upon output from the first light detector.

TECHNICAL FIELD

This disclosure is directed to the field of laser scanning projectorsand, in particular, to a scanning laser projector system utilizing oneor more photodiodes in the projector's scan area within the housing ofthe laser scanning projector, but outside of the projector's projectionarea, to provide feedback on laser beam angular position and laser beamoptical power.

BACKGROUND

A laser scanning projector is a small, portable electronic device. Laserscanning projectors are typically paired to, or incorporated within,user devices such as smart glasses, smartphones, tablets, laptops, ordigital cameras, and used to project virtual and augmented reality,documents, images, or video stored on those user devices onto aprojection surface, such as a wall, light field, holographic surface, orinner display surface of virtual or augmented reality glasses. Laserscanning projectors are also incorporated within distance determinationsystems within vehicles to determine information about the vehiclerelative its surroundings, permitting the creation of advanced driverassistance systems.

Such laser scanning projectors typically include a projection subsystemand an optical module. The paired user device serves an image stream(e.g., a video stream, or a pattern projected into an area used fordistance determination) to the projection subsystem. The projectionsubsystem properly drives the optical module to project the image streamonto the projection surface or environment for viewing.

In greater detail, typical optical modules are comprised of one or morelaser sources and one or more microelectromechanical (MEMS) mirrors thatscan the laser beam produced by the laser source across the projectionsurface in a projection pattern. By modulating the laser beam (in thecase of an image projection system) according to its position on theprojection surface, while the laser beam is scanned in the projectionpattern, the image stream is displayed. Commonly, at least one lensfocuses the beam before being reflected by the one or more MEMS mirrors,and then the laser beam strikes the projection surface or environment,although optical modules of other designs may be used.

The projection subsystem controls the driving of the laser source andthe driving of the movement of the one or more MEMS mirrors. Typically,the driving of movement of one of MEMS mirrors is at, or close to, theresonance frequency of that MEMS mirror, and the driving of movement ofanother of the MEMS mirrors is performed linearly and not at resonance,although there are projection subsystem architectures in which bothmirrors are driven close to their resonance frequency.

Monitoring of the mirror movement of the one or more MEMS mirrors isperformed by the projection subsystem, for example to determine theopening angle of the one or more MEMS mirrors. This is used as feedbackto maintain the opening angle at a desired value, and also to determinewhether a mirror malfunction has occurred. When a mirror malfunction hasoccurred, it is desired to immediately turn off the laser sources.Existing systems that perform monitoring of mirror movement may requireextensive calibration, which is not desirable because of increasedproduction time and cost. In addition, it is possible for failure ofmirror drive electronics to yield a false negative using existingsystems, leading to continued operation of a malfunctioning system.Thus, it would be desired for there to be a backup for mirror openingangle determination that does not fail when the mirror drive electronicsfail.

Typically, a power detector is placed within a path traveled by thelaser beam within the optical module prior to the laser beam reachingthe MEMS mirrors, with this power detector operating to detect the laserbeam and generate a signal based upon the laser beam. From this signal,information about the laser beam itself can be determined. For example,information about the spot size of the laser beam, luminosity of thelaser beam, color calibration of the laser beam, frequency of the laserbeam, and modulation speed of the laser beam can be determined. However,if such power detector or the circuitry reading the power detector andperforming the calculations to produce the desired data experiences afailure, this functionality would be lost, and the laser scanningprojector would fail. This, it is desired for there to be a backup powerdetection system. To accomplish this, it is conventional for thecircuitry reading the power detector and performing the calculations toproduce the desired data to be replicated, therefore providing analternative backup path, and permitting proper operation of the laserscanning projector. However, this increases cost, complexity, and areaconsumed by the electronics for the laser scanning projector.

As such, it would be advantageous if a system could be designed whichcan provide an optical opening angle monitoring as well as a backuppower detection system, while overcoming the drawbacks outlined above.As such, further development is needed.

SUMMARY

Disclosed herein is a scanning laser projector including an opticalmodule. The optical module includes a housing carrying: a lasergenerator configured to output a single laser beam; a movable mirrorapparatus; wherein the movable mirror apparatus is configured to scanthe single laser beam across an exit window defined through the housingin a scanning pattern wider than the exit window such that the singlelaser beam is directed through the exit window in a projection pattern,the projection pattern being smaller than and within the scanningpattern; and a first light detector positioned about a periphery of theexit window such that as the movable mirror apparatus scans the singlelaser beam in the scan pattern, at a point in the scan pattern where thesingle laser beam is scanned across an interior of the housing and notthrough the exit window, the single laser beam impinges upon the firstlight detector. A projection engine is configured to adjust driving ofthe movable mirror apparatus based upon output from the first lightdetector.

The movable mirror apparatus may include a first mirror position sensorgenerating output indicative of deflection of a first mirror within themovable mirror apparatus. The projection engine may include: a triggercircuit coupled to receive output from the first light detector and toassert a trigger signal in response to the output from the first lightdetector indicating that the single laser beam has impinged upon thefirst light detector; a first sample/hold circuit configured to sampleand hold output of the first mirror position sensor in response toassertion of the trigger signal as a first held value; an analog todigital converter configured to digitize the first held value to producea first digitized value; and a control circuit configured to adjustdriving of the movable mirror apparatus based upon the first digitizedvalue.

The control circuit may be configured to calibrate the first mirrorposition sensor based upon the first digitized value.

The control circuit may calibrate the first mirror position sensor basedupon the first digitized value using a priori knowledge of an expecteddeflection angle of the first mirror when the single laser beam impingesupon the first light detector.

The control circuit may calibrate the first mirror position sensor basedupon the first digitized value by: determining an instantaneous gain inoutput of the first mirror position sensor as a ratio of a calibrationvalue to the first digitized value; and scaling the first digitizedvalue to account for the instantaneous gain in the output of the firstmirror position sensor prior to adjusting driving of the movable mirrorapparatus based upon the first digitized value.

The projection engine may adjust the driving of the movable mirrorapparatus to maintain a first mirror within the movable mirror apparatusat a first opening angle, based upon the first digitized value.

The projection engine may be configured to cause the laser generator tocease outputting the single laser beam based upon lack of the triggercircuit asserting the trigger signal.

The projection engine may be configured to cause the laser generator tocease outputting the single laser beam based upon lack of the triggercircuit asserting the trigger signal when expected.

A second light detector may be positioned about the periphery of theexit window and on a different side thereof than the first lightdetector, the second light detector being located such that as themovable mirror apparatus scans the single laser beam in the scanpattern, at a point in the scan pattern where the single laser beam isscanned across an interior of the housing and not through the exitwindow, the single laser beam impinges upon the second light detector.The projection engine may adjust the driving of the movable mirrorapparatus based upon output from the first and second light detectors.

The movable mirror apparatus may include a second mirror position sensorgenerating output indicative of deflection of a second mirror within themovable mirror apparatus. The trigger circuit may also be coupled toreceive output from the second light detector and to assert the triggersignal in response to the output from the second light detectorindicating that the single laser beam has impinged upon the second lightdetector. The projection engine may further include a second sample/holdcircuit configured to sample and hold output of the second mirrorposition sensor in response to assertion of the trigger signal as asecond held value. The analog to digital converter may be furtherconfigured to digitize the second held value to produce a seconddigitized value, and wherein the control circuit may be furtherconfigured to adjust driving of the movable mirror apparatus based uponthe second digitized value.

The control circuit may be configured to calibrate the first mirrorposition sensor based upon the first digitized value and calibrate thesecond mirror position sensor based upon the second digitized value.

The control circuit may calibrate the first mirror position sensor basedupon the first digitized value using a priori knowledge of an expecteddeflection angle of the first mirror when the single laser beam impingesupon the first light detector and calibrates the second mirror positionsensor based upon the second digitized value using a priori knowledge ofan expected deflection angle of the second mirror when the single laserbeam impinges upon the second light detector.

The control circuit may calibrate the first mirror position sensor basedupon the first digitized value by: determining an instantaneous gain inoutput of the first mirror position sensor as a ratio of a firstcalibration value to the first digitized value; and scaling the firstdigitized value to account for the instantaneous gain in the output ofthe first mirror position sensor prior to adjusting driving of themovable mirror apparatus based upon the first digitized value. Thecontrol circuit may calibrate the second mirror position sensor basedupon the second digitized value by: determining an instantaneous gain inoutput of the second mirror position sensor as a ratio of a secondcalibration value to the second digitized value; and scaling the seconddigitized value to account for the instantaneous gain in the output ofthe second mirror position sensor prior to adjusting driving of themovable mirror apparatus based upon the second digitized value.

The control circuit may be configured to generate a synchronizationsignal for the first mirror or the second mirror based upon an elapsedtime between detections of the single laser beam by the first lightdetector and then by the second light detector.

The projection engine may adjust the driving of the movable mirrorapparatus so as to maintain a first mirror within the movable mirrorapparatus at a first opening angle, based upon the output from the firstlight detector.

The projection engine may be configured to cause the laser generator tocease outputting the single laser beam based upon the output from thefirst light detector indicating that the single laser beam did notimpinge upon the first light detector.

The projection engine may be configured to cause the laser generator tocease outputting the single laser beam based upon the output from thefirst light detector indicating that the single laser beam did notimpinge upon the first light detector when expected.

The projection engine may be configured to cause the laser generator tomodify color and intensity of the single laser beam based upon theoutput from the first light detector.

Also disclosed herein is an optical module including a housing, thehousing carrying: a laser generator configured to output a single laserbeam; a movable mirror apparatus; wherein the movable mirror apparatusis configured to scan the single laser beam across an exit windowdefined through the housing in a scanning pattern wider than the exitwindow such that the single laser beam is directed through the exitwindow in a projection pattern, the projection pattern being smallerthan and within the scanning pattern; a first light detector positionedabout a periphery of the exit window such that as the movable mirrorapparatus scans the single laser beam in the scan pattern, at a point inthe scan pattern where the single laser beam is scanned across aninterior of the housing and not through the exit window, the singlelaser beam impinges upon the first light detector; and a second lightdetector positioned about the periphery of the exit window and on adifferent side thereof than the first light detector, the second lightdetector being located such that as the movable mirror apparatus scansthe single laser beam in the scan pattern, at a point in the scanpattern where the single laser beam is scanned across an interior of thehousing and not through the exit window, the single laser beam impingesupon the second light detector.

The first and second light detectors may be located on opposite sides ofthe periphery of the exit window, outside of the projection pattern butinside the scanning pattern.

The first and second light detectors may be located on opposite sides ofa first axis that bisects the exit window.

The first and second light detectors may also be located on oppositesides of a second axis perpendicular to the first axis.

Also disclosed herein is a method of operating a laser scanningprojector contained within a housing, the method including: generating asingle laser beam; scanning the single laser beam across an exit windowdefined through the housing in a scanning pattern wider than the exitwindow such that the single laser beam is directed through the exitwindow in a projection pattern, the projection pattern being smallerthan and within the scanning pattern; at points in the scan patternwhere the single laser beam is scanned across an interior of the housingand not through the exit window, detecting the single laser beam as itimpinges upon a first light detector positioned about a periphery of theexit window; and adjusting the scanning of the laser beam based upon theimpinging of the single laser beam upon the first light detector.

The scanning of the single laser beam may be accomplished via deflectionof a movable mirror apparatus. Adjusting the scanning of the laser beambased upon the impinging of the single laser beam upon the first lightdetector may include: in response to the impinging of the single laserbeam upon the first light detector, digitizing a mirror position signalsupplied by a mirror position sensor associated with the movable mirrorapparatus, and adjusting movement of the movable mirror apparatus basedupon the digitized mirror position signal.

The method may include calibrating the mirror position sensor based uponthe digitized mirror position signal.

The method may include correcting the digitized mirror position signalbased upon the calibration of the mirror position sensor.

The method may further include determining an instantaneous gain inoutput of the mirror position sensor as a ratio of a calibration valueto the digitized mirror position signal, and scaling the digitizedmirror position signal to account for the instantaneous gain in theoutput of the mirror position sensor prior to adjusting movement of themovable mirror apparatus based upon the digitized mirror positionsignal.

The method may further include ceasing generation of the single laserbeam based upon lack of impinging of the single laser beam upon thefirst light detector.

The method may further include ceasing generation of the single laserbeam based upon lack of impinging of the single laser beam upon thefirst light detector at an expected time.

The method may further include adjusting the generation of the singlelaser beam based upon output from the first light detector.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatical representation of an optical module disclosedherein.

FIG. 2 is a diagrammatical representation of the scanning area andprojection area defined by the movement of the mirrors of the opticalmodule of FIG. 1 .

FIG. 3 is a perspective view of components of the optical module of FIG.1 .

FIG. 4 is diagrammatical representation of the vertical mirror,horizontal mirror, and folding mirror of FIG. 1 .

FIG. 5 is a diagrammatical front elevational view of the vertical mirrorof FIG. 1 , and the scanning area and projection area defined by themovement of the horizontal mirror of FIG. 1 .

FIG. 6 is a diagrammatical front view of the vertical mirror of FIG. 1 ,showing possible placements for the photodiodes located within thescanning area but outside of the projection area.

FIG. 7 is a diagrammatical representation of the scanning area andprojection area defined by the movement of the mirrors of the opticalmodule of FIG. 1 , including potential positions for the photodiodeslocated within the scanning area but outside of the projection area.

FIG. 8A is a first embodiment of a block diagram of a scanning laserprojector including the optical module of FIG. 1 .

FIG. 8B is a second embodiment of a block diagram of a scanning laserprojector including the optical module of FIG. 1 .

FIG. 9 is a diagrammatical representation of the scanning area andprojection area defined by the movement of the mirrors of the opticalmodule of FIG. 1 , including potential positions for the photodiodeslocated within the scanning area but outside of the projection area, andshowing the mirror drive signals.

DETAILED DESCRIPTION

The following disclosure enables a person skilled in the art to make anduse the subject matter disclosed herein. The general principlesdescribed herein may be applied to embodiments and applications otherthan those detailed above without departing from the spirit and scope ofthis disclosure. This disclosure is not intended to be limited to theembodiments shown, but is to be accorded the widest scope consistentwith the principles and features disclosed or suggested herein.

An optical module 10 is now described with reference to FIG. 1 . Theoptical module 10 includes a housing 11 carrying a compact RGB laserpackage 12 that includes a red laser diode 12 a, green laser diode 12 b,and blue laser diode 12 c therein.

Alignment lenses 14 a, 14 b, and 14 c are carried within the housing 11adjacent the RGB laser package 12, and serve to collimate the laserbeams 30, 31, and 32 respectively generated by the red laser diode 12 a,green laser diode 12 b, and blue laser diode 12 c in operation. Thealignment lenses 14 a, 14 b, and 14 c are set such that the laser spotswould overlap at a certain distance, for example, at a 450 mm focaldistance. In addition, the maximum angular deviation between any twolaser spots may helpfully be no more than 0.2°, and the maximumdeviation between all laser spots should helpfully be no more than 0.5°.The spot size produced by the red laser diode 12 a, after focusing bythe alignment lens 14 a, may be around 830×650 microns; the spot sizeproduced by the blue laser diode 12 b, after focusing by the alignmentlens 14 b, may be around 800×600 microns; and the spot size produced bythe green laser diode 12 c, after focusing by the alignment lens 14 c,may be around 780×550 microns. If the focal distance is changed fromthis example for a particular application, the spot size changesaccordingly. The alignment lenses 14 a, 14 b, and 14 c may have anumerical aperture of 0.38, with an effective focal length of 2 mm, anda 1 mm diameter, and may be coated with anti-reflective coating thatallows light in the range of 400 nm-700 nm to pass but rejects otherlight. The alignment lenses 14 a, 14 b, and 14 c may have a generallycylindrical cross section, with a flat rear surface and a convex frontsurface, or, in some cases, may have an aspherical shape. The effectivefocal length and diameter of the alignment lenses 14 a, 14 b, and 14 ccan be altered as desired for specific applications. For example, thealignment lenses 14 a, 14 b, and 14 c may be 1.5 mm in diameter. It willalso be appreciated that in some cases, the alignment lenses 14 a, 14 b,and 14 c may have different diameters from one another, or one of thealignment lenses may have a different diameter than the other twoalignment lenses.

A 4:1 beam splitter 16 is carried within the housing 11 adjacent thealignment lenses 14 a, 14 b, and 14 c. This beam splitter 16 is a singlerectangularly shaped unit formed of three square units, each square unitbeing comprised of two triangular prisms having their bases affixed toone another. The overall dimensions of the beam splitter may be, forexample, 6 mm in length, 2 mm in depth, and 2.5 mm in height. Naturally,these dimensions are just examples, and the beam splitter 16 may insteadhave other dimensions, and individual lenses may be used instead of thebeam splitter.

The prisms of the beam splitter 16 that serve to reflect the laser beams30 and 31 are arranged so as to reflect as close to 100% of those beamsas possible along a trajectory out the right side of the beam splitter36 to help form the combined RGB laser beam 33, while the prisms of thebeam splitter 16 that serve to reflect the laser beam 32 are arranged soas to reflect about 98% of the laser beam 32 out the right side of thebeam splitter 36 to form the combined RGB laser beam 33, while passingabout 2% of the laser beam 32 through to reach a photodiode 18 used toprovide feedback for the system driving the laser diodes 12 a, 12 b, and12 c of the RGB laser package 12.

While the beam splitter 16 here is used to combine the laser beams 30,31, and 32 to form the RGB laser beam 33, the beam splitter 16 is stilltechnically a 4:1 beam splitter, as if a beam 33 were to be input intothe right side (the output) of the beam splitter 16, the beam splitterwould split it to produce the beams 32 (exiting toward the lens 14 c andtoward the photodiode 18), 31, and 30. Thus, despite its use as a beamcombiner, the component 16 is indeed a beam splitter 16.

A vertical mirror 20, a horizontal mirror 24, and a folding mirror 22are adjacent the beam splitter 16, and collectively are used to reflectthe RGB laser beam 33 out an exit window 26 on a housing 11 and onto adisplay surface. Note that the position of the folding mirror 22 isfixed during operation, while the horizontal mirror 24 is driven tooscillate at its resonance frequency and the vertical mirror 20 isdriven linearly. Therefore, the purpose of the folding mirror 22 issimply to “fold” the path of the RGB laser beam 33 to strike thehorizontal mirror 24, while the purpose of the horizontal mirror 24 andvertical mirror 20 is to scan the RGB laser beam 33 across the displaysurface in a scan pattern designed to reproduce the desired still ormoving images. The total area scanned by the horizontal mirror 24 andvertical mirror 20 defines a scanning area, as shown in FIG. 2 ;similarly, the pattern formed by the laser beam during the portion ofthe scanning area during which the laser beam 33 exits the exit window26 defines a projection area, as shown in FIG. 2 . Thus, understand thatthe laser beam 33 may be scanned across portions of the inside surfaceof the housing wall 11 (on the side thereof where the exit window 26 islocated). As a result, the scanning area is larger the projection area,and the projection area is included within the scanning area, as seen inFIG. 2 .

The overall dimensions of the vertical mirror 20 may be, for example,7.94 mm in length, 2.34 mm in depth, and 0.67 mm in height; the overalldimensions of the horizontal mirror 24 may be, for example, 4.44 mm inlength, 2.94 mm in depth, and 0.67 mm in height. Naturally, the verticalmirror 20 and horizontal mirror 24 may have other dimensions, and thegiven dimensions are just examples.

Turning now to FIG. 3 , the geometry of the vertical mirror 20,horizontal mirror 24, and folding mirror 22 is now described in thecontext of the optical module 10. The RGB laser beam 33 is aimed by thebeam splitter 16 to pass over the top of the vertical mirror 20 tostrike the folding mirror 22, which reflects the RGB laser beam 33 ontothe horizontal mirror 24, which then reflects the RGB laser beam 33 ontothe vertical mirror 20, which reflects the RGB laser beam 33 out theexit window 26 on the housing 11 and onto the display surface. The beamsplitter 16 splits the RGB laser beam 33 such that a portion of the RGBlaser beam 33 is reflected to strike the photodiode 18 within theoptical module 10, and the output of the photodiode 18 is used asfeedback for control of the RGB laser beam 33 and the laser beams 30,31, and 32 that are combined to form the RGB laser beam 33.

Sample angles for this path taken by the RGB laser beam 33 may be seenin FIG. 4 , where the folding mirror 32 reflects the RGB laser beam 33at an angle of 54° toward the horizontal mirror 24, and the horizontalmirror 24 reflects the RGB laser beam 33 at an angle of 54° toward thevertical mirror. The vertical mirror 20 is arranged to reflect the RGBlaser beam 33 in a direction parallel to the plane in which thehorizontal mirror 24 lies, and therefore directly out the exit window 26without any keystone. In this arrangement, it may be observed that thepath traveled by the RGB laser beam 33 between the centers of thehorizontal mirror 24 and vertical mirror 20 is about 1.02 mm. Noticehere that the opening angle of the vertical mirror 20 is 11°, definingthe scanning area in the vertical direction. However, also see that theprojection area in the vertical direction is less than this, and that,for example, the projection area in the vertical direction is defined by9° degrees of the opening angle of the vertical mirror 20.

Referring now to FIG. 5 , it can be observed that the opening angle ofthe horizontal mirror 24 is 18°, defining the scanning area in thehorizontal direction. Also observe that the projection area in thehorizontal direction is less than this, and that, for example, theprojection area in the horizontal direction is defined by 16° of theopening angle of the vertical mirror, with the aspect ratio of theresulting projection area being 16:9, for example.

First, the position of additional photodiodes 18 a and 18 b within thehousing 11 will be described in detail, as will be the position ofadditional other photodiodes, and then the usage for those photodiodes18 a and 18 b will be described in detail. However, in general,understand that the photodiodes 18 a, 18 b, and other photodiodes withinthe housing, may be used as a backup for the photodiode 18, and may beused to collect data to be used for monitoring and modifying operationof the laser scanning projector 10.

As shown in FIGS. 1, 3, 4, and 5 , in addition to the photodiode 18being used to provide feedback about the laser beam 33, photodiodes 18 aand 18 b are positioned within the housing 11 on opposite sides of theexit window 26. As is well shown in FIGS. 3-4 , the photodiodes 18 a and18 b are positioned outside of the projection area but within thescanning area; the photodiodes 18 a and 18 b are also positioneddownstream of the mirrors 20 and 24, meaning that it is the light thatis reflected by the mirrors 20 and 24 that reaches the photodiodes 18 aand 18 b. The photodiodes 18 a and 18 b do not occlude any portion ofthe projection area, but when the laser beam 33 is scanned across thescanning area outside of the projection area, the photodiodes 18 a and18 b detect the laser beam 33 when it impinges upon them. In theexamples shown in FIGS. 1 and 3-4 , the photodiodes 18 a and 18 b arelocated on opposite vertical sides of the exit window 26, meaning thatwhen the laser beam 33 impinges upon the photodiode 18 a it has movedoutside of the projection area in the positive vertical direction, andwhen the laser beam 33 impinges upon the photodiode 18 b it has movedoutside of the projection area in the negative vertical direction.

As shown in FIG. 5 , photodiodes 18 c and 18 b are positioned within thehousing on opposite sides of the exit window. As is well shown in FIG. 4, the photodiodes 18 c and 18 b are positioned outside of the projectionarea but within the scanning area, and therefore do not occlude theprojection area. However, when the laser beam 33 is scanned across thescanning area outside of the projection area, the photodiodes 18 c and18 d detect the laser beam 33 when it impinges upon them. In the exampleshown in FIG. 5 , the photodiodes 18 c and 18 d are located on oppositehorizontal sides of the exit window 26, meaning that when the laser beam33 impinges upon the photodiode 18 c it has moved outside of theprojection area in the negative horizontal direction, and when the laserbeam 33 impinges upon the photodiode 18 d it has moved outside theprojection area in the positive horizontal direction.

Although between FIGS. 1 and 3-5 the use of four photodiodes 18 a-18 dis shown, understand that more such photodiodes may be used. Forexample, shown in FIG. 6 is an example where there are eight suchphotodiodes 18 a-18 g are shown. Less than eight photodiodes, and as fewas two photodiodes may be used, with possible positions for each diodeused being one of the positions shown in FIG. 6 .

In the above examples, the photodiodes 18 a-18 d were shown as beingplaced along the horizontal and vertical axis of the projection area,but it should be understood that this need not be the case. For example,see the case shown in FIG. 7 , where photodiode 18 a is located to theright of the vertical axis of the projection area, photodiode 18 b islocated to the left of the vertical axis of the projection area,photodiode 18 c is located above the horizontal axis of the projectionarea, and photodiode 18 d is located below the horizontal axis of theprojection area.

Details of a laser scanning projector 40 incorporating the opticalmodule 10 and a projection engine 41, utilizing the photodiodes 18 a and18 b within the optical module 10 for feedback, are now given withreference to FIG. 8A. The optical module 10 includes the componentsdescribed above, with here the RGB laser package 12, vertical mirror 20,and horizontal mirror 24 being shown, but the lenses 14 a-14 c, beamsplitter 16, folding mirror 22, and exit window 26 not being shown forsimplicity.

Referring now to the optical module 10, the vertical mirror 20 has atilt sensor 44 associated therewith to measure deflection or tilt of thevertical mirror 20. The tilt sensor 44 may be piezoelectric orpiezoresistive, and provides a signal V-Sense indicative of thedeflection or tilt of the vertical mirror 20 to the projection engine41. The horizontal mirror 24 has a similar tilt sensor 45 associatedtherewith to measure deflection or tilt of the horizontal mirror 24, andprovides a signal H-Sense indicative of the deflection or tilt of thehorizontal mirror 20 to the projection engine 41. The vertical mirror 20and horizontal mirror 24 are driven by mirror drivers 48, with thevertical mirror 24 being driven at resonance and the horizontal mirror20 being driven linearly. Laser drivers 47 drive the RGB laser package12 to cause the lasers within the RGB laser package 12 to lase atdesired times and desired intensities.

The projection engine 41 is now described. The projection engine 41includes a sample/hold circuit 51 that receives the signal generated bythe tilt sensor 44 of the vertical mirror 20, and a sample/hold circuit52 that receives the signal generated by the tilt sensor 45 of thehorizontal mirror 44. An analog-to-digital converter (ADC) 43 receivesthe values held by the sample/hold circuits 51 and 52, digitizes thosevalues, and provides them to a control circuit 46. The photodiode 18 isalso fed to the ADC 13, which provides a digitized version of thephotodiode 18 output to the control circuit 46, which generates controlsignals for the mirror drivers 48 and the laser drivers 47 based partlyon the photodiode 18, and on the photodiodes 18 a and 18 b. A triggercircuit 42 receives output from the photodiodes 18 a and 18 b (as wellas any other photodiodes such as photodiodes 18 c-18 g present withinthe housing 11), and triggers the sample/hold circuits 51 and 52 tosample and hold the signal at their inputs when the photodiodes detectimpinging laser light.

Operation is now described. As explained earlier, the mirror drivers 48drive the vertical mirror 24 at resonance and drive the horizontalmirror 20 linearly while the RGB package 12 generates and modulates thelaser beam 33 accordingly to form a desired image or to scan a scene forperforming depth sensing. As explained above, the laser beam 33 isscanned (reflected) by the mirrors in a pattern. In particular, thedriving of the mirrors 20 and 24 serves to scan the laser beam 33 in ascan pattern in a scanning area, shown for example in FIG. 2 . Thephotodiode 18, as explained above is within the beam generation pathinside the optical module 10 as shown in FIGS. 1 and 3 , and providesfeedback to the control circuit 46, which in turn modifies its controlof the mirror drivers 48 and laser drivers 47 based upon that feedback.In particular, the intensity, color, and modulation of the laser can bemodified by the control circuit 46 based upon the feedback provided bythe photodiode 18.

As explained above, the scanning area is larger than the projectionarea, meaning that as the mirrors 20 and 24 scan the laser beam 33across the portion of the scanning area outside of the projection area,they are scanning the laser beam 33 across the inside wall of thehousing 11 of the optical module 10 adjacent the location of the window26.

When the laser beam 33 scans across the photodiode 18 a, for example,the output signal generated by the photodiode 18 a is received by thetrigger circuit 42, which in turn activates the sample/hold circuit 51.When activated, the sample/hold circuit 51 samples and holds the currentvalue of V-sense, which is the output of the tilt sensor 44 for thevertical mirror 20. Similarly, when the laser beam 33 scans across thephotodiode 18 b, the output signal generated by the photodiode 18 b isreceived by the trigger circuit 42, which in turn activates thesample/hold circuit 52. When activated, the sample/hold circuit 52samples and holds the current value of H-sense, which is the output ofthe tilt sensor 45 for the horizontal mirror 24. See, for example, FIG.9 , showing the data points of V-Sense and H-Sense captured when thephotodiodes 18 a and 18 b are impinged upon by the laser beam 33. TheADC 43 digitizes the values of V-Sense and H-Sense stored by thesample/hold circuits 51 and 52, and passes those values to the controlcircuit 46.

The sampled values of V-Sense and H-Sense respectively taken when thephotodiodes 18 a and 18 b are impinged upon by the laser beam 33 can beused by the control circuit 46 for modifying control of the mirrordrivers 48 so as to control the opening angle of the vertical mirror 20and horizontal mirror 24. In greater detail, when the photodiodes 18 aand 18 b are impinged upon by the laser beam 33, it is known from priorcalibration what the deflection angles/mechanical angles of the verticalmirror 20 and horizontal mirror 24 are at those two instants. Theseknown angles of the vertical mirror 20 and horizontal mirror 24 can becorrelated to the signals V-Sense and H-Sense from the tilt sensors 44and 45 to calibrate the tilt sensors 44 and 45 on the fly. Since thetilt sensors 44 and 45 are calibrated on the fly, this means thatV-Sense and H-Sense can be used by the control circuit 46 in adjustingthe mirror drivers 48 to a high degree of accuracy so as to maintaindesired opening angles of the vertical mirror 20 and horizontal mirror24 during operation. In particular, the control circuit 46 can determinecorrections to apply to the raw V-Sense and H-Sense signals when usingthose signals in controlling the mirror drivers 48, based upon thesamples of V-Sense and H-Sense taken when the vertical mirror 20 andhorizontal mirror 24 are at known deflection/mechanical angles asindicated by triggering of the photodiodes 18 a and 18 b. As usedherein, an opening angle of a mirror means the angle between its maximumpositive deflection or mechanical angle and its maximum negativedeflection or mechanical angle.

The above will now be mathematically described. In a calibrationoperation performed as a one-time setup during device fabrication,calibration values A₀ and B₀ can respectively be determined for the tiltsensors 44 and 45.

Calibration signal A₀ for the tilt sensor 44 can be determined duringthe calibration operation as a difference between the value A₁ ⁰ ofV-Sense when the laser beam 33 impinges upon the photodiode 18 a and thevalue A₂ ⁰ of V-Sense when the laser beam 33 impinges upon thephotodiode 18 b. Mathematically, this calculation can be represented as:A ₀ =A ₁ ⁰ −A ₂ ⁰

Calibration signal B₀ for the tilt sensor 45 can be determined duringthe calibration operation as a difference between the value B₁ ⁰ ofH-Sense when the laser beam 33 impinges upon the photodiode 18 a and thevalue B₂ ⁰ of H-Sense when the laser beam 33 impinges upon thephotodiode 18 b. Mathematically, this calculation can be represented as:B ₀ =B ₁ ⁰ −B ₂ ⁰Keeping this in mind, during operation, a present value A(t) for thetilt sensor 44 can be determined as a difference between the value A₁(t)of V-Sense when the laser beam 33 impinges upon the photodiode 18 a andthe value A₂(t) of V-Sense when the laser beam 33 impinges upon thephotodiode 18 b. Mathematically, this calculation can be represented as:A(t)=A ₁(t)−A ₂(t)

From A₀ and A(t), an instantaneous gain GA(t) for the tilt sensor 44 canbe calculated. Mathematically, this can be represented as:G(t)=A ₀ /A(t)

Likewise, a present value B(t) for the tilt sensor 45 can be determinedas a difference between the value B₁(t) of H-Sense when the laser beam33 impinges upon the photodiode 18 a and the value B₂(t) of H-Sense whenthe laser beam 33 impinges upon the photodiode 18 b. Mathematically,this calculation can be represented as:B(t)=B ₁(t)−B ₂(t)

From B₀ and B(t), an instantaneous gain GB(t) for the tilt sensor 44 canbe calculated. Mathematically, this can be represented as:GB(t)=B ₀ /B(t)

Since A(t) and B(t) may vary over temperature, this means that theinstantaneous gains GA(t) and GB(t) may vary over temperature.Therefore, by the control circuit 46 knowing the instantaneous gainsGA(t) and GB(t), it can correct or scale the values of V-Sense andH-Sense to produce corrected or scaled values of V-Sense and H-Sensethat are temperature independent, and its regulation of the mirrordrivers 48 can be performed using the corrected or scaled values insteadof the raw values which may be temperature dependent.

An advantage provided by this system is that the tilt sensors 44 and 45do not need to be carefully calibrated over temperature at the time ofdevice fabrication, as the on-the-fly calibration of the tilt sensors 44and 45 performed by the control circuit 46 using the sampled values ofV-Sense and H-Sense triggered by the photodiodes 18 a and 18 bcompensates for temperature.

Further operation is enabled through the use of the photodiodes 18 a and18 b. For example, since the control circuit 46 operates the mirrordrivers 48, the control circuit 46 has a priori knowledge of when toexpect the trigger circuit 42 to be triggered by the laser beam 33impinging upon the photodiodes 18 a and 18 b. Thus, if the triggeringdoes not occur, the control circuit 46 can assume that failure of acomponent has occurred and can cause the laser drivers 47 to stopdriving the RGB package 12. The response time for this is equal to atime frame (expected time interval between two successive triggerings ofthe trigger circuit 42) divided by the number of tilt sensors,represented mathematically as:T _(frame) /M _(sensors)

This fault detection is particularly useful because it can detect mirrorfailures not detected by the tilt sensors 44 and 45. For example, it ispossible for a tilt sensor to break a torsional spring, and thereforereport on movement of the piezoelectric actuator within the tilt sensorand not the mirror movement. Thus, this fault detection system can beused in addition to normal fault detection, or instead of normal faultdetection.

Also note that through receipt of digitized versions of the outputs ofthe photodiodes 18 a and 18 b themselves, as shown in the embodiment ofFIG. 8B, the control circuit 46 may perform additional functionality.For example, referring back to FIG. 7 , regardless of where photodiode18 a is with respect to the vertical axis of the projection area, sinceit is located outside of the projection area (but inside the scanningarea) and above the projection area, when the photodiode 18 a isimpinged upon by the laser, it can be inferred (explained in greatdetail below) that the laser beam 33 has successfully exited theprojection area in the positive vertical direction; regardless of wherephotodiode 18 b is with respect to the vertical axis of the projectionarea, since it is located outside of the projection area (but inside thescanning area) and below the projection area, when the photodiode 18 bis impinged upon by the laser, it can be inferred that the laser beam 33has successfully exited the projection area in the negative verticaldirection; regardless of where photodiode 18 c is with respect to thehorizontal axis of the projection area, since it is located outside ofthe projection area (but inside the scanning area) and to the left ofthe projection area, when the photodiode 18 c is impinged upon by thelaser, it can be inferred that the laser beam 33 has successfully exitedthe projection area in the negative horizontal direction; and regardlessof where photodiode 18 d is with respect to the horizontal axis of theprojection area, since it is located outside of the projection area (butinside the scanning area) and to the right of the projection area, whenthe photodiode 18 d is impinged upon by the laser, it can be inferredthat the laser beam 33 has successfully exited the projection area inthe positive horizontal direction. Therefore, in addition to determiningthat a mirror error has occurred, the control circuit 46 can determinewhich mirror has failed, and in what direction, if desired.

In addition, the control circuit 46 can use feedback from thephotodiodes 18 a and 18 b to control the color and intensity/brightnessof the laser beam 33. Also, the control circuit 46 can use feedback fromthe photodiodes 18 a and 18 b to measure optical attenuation of thelaser beam 33 due to the mirrors, for example by comparing color andintensity data from the photodiodes 18 a and 18 b to color and intensitydata from the photodiode 18 (since the photodiode 18 measures the laserbefore reflection by the vertical mirror 20 and horizontal mirror 24).

Also, the control circuit 46 can use feedback from the photodiodes 18 aand 18 b to determine failure of the vertical mirror 20 and/orhorizontal mirror 24 (without use of the trigger circuit 22) based uponthe photodiodes 18 a and 18 b not being triggered according to thevelocity of the swing of those mirrors. In the case of the horizontalmirror 24, which is driven at resonance, it can be expected that eachphotodiode 18 a and 18 b should be triggered at half the mirrorswing—for example, a mirror operating at a 20 KHz resonance frequencywill complete one full swing every 50 μs, and therefore, each photodiode18 a and 18 b should detect the laser beam 33 every 25 μs. In the caseof the vertical mirror 20, driven linearly, the velocity is constant andtherefore each photodiode 18 a and 18 b is triggered at half mirrordrive frequency. For example, a mirror operating at 60 Hz linearly willcomplete its entire swing in 16 ms, so each photodiode 18 a and 18 bshould detect the laser beam 33 every 8 ms. In case either photodiode 18a and 18 b does not detect the laser beam 33 at the expected time,mirror malfunction can be inferred.

Still further, the control circuit 46 can generate horizontal andvertical synchronization signals for use in controlling the mirrordrivers 48. For example, by detecting the elapsed time betweenactivation of the photodiode 18 a by the laser beam 33 and activation ofthe photodiode 18 b by the laser beam 33, such horizontal and verticalsynchronization signals can be calculated.

In the above, photodiodes 18, and 18 a-18 g may actually be any lightdetecting devices, and may each be a one-dimensional array ortwo-dimensional array of photodiodes. Also, although the abovedescriptions of functionality are described with reference to thephotodiodes 18 a and 18 b being present in addition to the photodiode18, more such photodiodes (for example, photodiodes 18 c, 18 d, etc.)may be present to provide for more data points. Also, as describedabove, any position for such photodiodes about the interior surface ofthe housing 11 adjacent the exit window 26 may be used provided that atleast two photodiodes are not on the same side of the exit window 26.

Finally, it is clear that modifications and variations may be made towhat has been described and illustrated herein, without therebydeparting from the scope of this disclosure, as defined in the annexedclaims.

While the disclosure has been described with respect to a limited numberof embodiments, those skilled in the art, having benefit of thisdisclosure, will appreciate that other embodiments can be envisionedthat do not depart from the scope of the disclosure as disclosed herein.Accordingly, the scope of the disclosure shall be limited only by theattached claims.

The invention claimed is:
 1. A scanning laser projector, comprising: anoptical module comprising a housing carrying: a laser generatorconfigured to output a single laser beam; a movable mirror apparatusincluding a first mirror position sensor generating output indicative ofdeflection of a first mirror within the movable mirror apparatus;wherein the movable mirror apparatus is configured to scan the singlelaser beam across an exit window defined through the housing in ascanning pattern wider than the exit window such that the single laserbeam is directed through the exit window in a projection pattern, theprojection pattern being smaller than and within the scanning pattern;and a first light detector positioned about a periphery of the exitwindow such that as the movable mirror apparatus scans the single laserbeam in the scanning pattern, at a point in the scanning pattern wherethe single laser beam is scanned across an interior of the housing andnot through the exit window, the single laser beam impinges upon thefirst light detector; and a projection engine configured to adjustdriving of the movable mirror apparatus based upon output from the firstlight detector by; wherein the projection engine comprises: a triggercircuit coupled to receive output from the first light detector and toassert a trigger signal in response to the output from the first lightdetector indicating that the single laser beam has impinged upon thefirst light detector; a first sample/hold circuit configured to sampleand hold output of the first mirror position sensor in response toassertion of the trigger signal as a first held value; an analog todigital converter configured to digitize the first held value to producea first digitized value; and a control circuit configured to adjustdriving of the movable mirror apparatus based upon the first digitizedvalue.
 2. The scanning laser projector of claim 1, wherein the controlcircuit is configured to calibrate the first mirror position sensorbased upon the first digitized value.
 3. The scanning laser projector ofclaim 2, wherein the control circuit calibrates the first mirrorposition sensor based upon the first digitized value using a prioriknowledge of an expected deflection angle of the first mirror when thesingle laser beam impinges upon the first light detector.
 4. Thescanning laser projector of claim 2, wherein the control circuitcalibrates the first mirror position sensor based upon the firstdigitized value by: determining an instantaneous gain in output of thefirst mirror position sensor as a ratio of a calibration value to thefirst digitized value; and scaling the first digitized value to accountfor the instantaneous gain in the output of the first mirror positionsensor prior to adjusting driving of the movable mirror apparatus basedupon the first digitized value.
 5. The scanning laser projector of claim1, wherein the control circuit adjusts the driving of the movable mirrorapparatus to maintain the first mirror within the movable mirrorapparatus at a first opening angle, based upon the first digitizedvalue.
 6. The scanning laser projector of claim 1, wherein the controlcircuit is configured to cause the laser generator to cease outputtingthe single laser beam based upon lack of the trigger circuit assertingthe trigger signal.
 7. The scanning laser projector of claim 1, whereinthe control circuit is configured to cause the laser generator to ceaseoutputting the single laser beam based upon lack of the trigger circuitasserting the trigger signal when expected.
 8. The scanning laserprojector of claim 1, further comprising a second light detectorpositioned about the periphery of the exit window and on a differentside thereof than the first light detector, the second light detectorbeing located such that as the movable mirror apparatus scans the singlelaser beam in the scanning pattern, at a point in the scanning patternwhere the single laser beam is scanned across the interior of thehousing and not through the exit window, the single laser beam impingesupon the second light detector; and wherein the control circuit adjuststhe driving of the movable mirror apparatus based upon output from thefirst and second light detectors.
 9. The scanning laser projector ofclaim 1, wherein the control circuit adjusts the driving of the movablemirror apparatus so as to maintain a first mirror within the movablemirror apparatus at a first opening angle, based upon the output fromthe first light detector.
 10. The scanning laser projector of claim 1,wherein the control circuit is configured to cause the laser generatorto cease outputting the single laser beam based upon the output from thefirst light detector indicating that the single laser beam did notimpinge upon the first light detector.
 11. The scanning laser projectorof claim 1, wherein the control circuit is configured to cause the lasergenerator to cease outputting the single laser beam based upon theoutput from the first light detector indicating that the single laserbeam did not impinge upon the first light detector when expected. 12.The scanning laser projector of claim 1, wherein the control circuit isconfigured to cause the laser generator to modify color and intensity ofthe single laser beam based upon the output from the first lightdetector.
 13. The scanning laser projector of claim 8, wherein themovable mirror apparatus includes a second mirror position sensorgenerating output indicative of deflection of a second mirror within themovable mirror apparatus; wherein the trigger circuit is also coupled toreceive output from the second light detector and to assert the triggersignal in response to the output from the second light detectorindicating that the single laser beam has impinged upon the second lightdetector; wherein the projection engine further comprises a secondsample/hold circuit configured to sample and hold output of the secondmirror position sensor in response to assertion of the trigger signal asa second held value; wherein the analog to digital converter is furtherconfigured to digitize the second held value to produce a seconddigitized value; and wherein the control circuit is further configuredto adjust driving of the movable mirror apparatus based upon the seconddigitized value.
 14. The scanning laser projector of claim 13, whereinthe control circuit is configured to calibrate the first mirror positionsensor based upon the first digitized value and calibrate the secondmirror position sensor based upon the second digitized value.
 15. Thescanning laser projector of claim 14, wherein the control circuitcalibrates the first mirror position sensor based upon the firstdigitized value using a priori knowledge of an expected deflection angleof the first mirror when the single laser beam impinges upon the firstlight detector and calibrates the second mirror position sensor basedupon the second digitized value using a priori knowledge of an expecteddeflection angle of the second mirror when the single laser beamimpinges upon the second light detector.
 16. The scanning laserprojector of claim 14, wherein the control circuit calibrates the firstmirror position sensor based upon the first digitized value by:determining an instantaneous gain in output of the first mirror positionsensor as a ratio of a first calibration value to the first digitizedvalue; and scaling the first digitized value to account for theinstantaneous gain in the output of the first mirror position sensorprior to adjusting driving of the movable mirror apparatus based uponthe first digitized value; and wherein the control circuit calibratesthe second mirror position sensor based upon the second digitized valueby: determining an instantaneous gain in output of the second mirrorposition sensor as a ratio of a second calibration value to the seconddigitized value; and scaling the second digitized value to account forthe instantaneous gain in the output of the second mirror positionsensor prior to adjusting driving of the movable mirror apparatus basedupon the second digitized value.
 17. The scanning laser projector ofclaim 13, wherein the control circuit is configured to generate asynchronization signal for the first mirror or the second mirror basedupon an elapsed time between detections of the single laser beam by thefirst light detector and then by the second light detector.
 18. Anoptical module, comprising a housing carrying: a laser generatorconfigured to output a single laser beam; a movable mirror apparatus;wherein the movable mirror apparatus is configured to scan the singlelaser beam across an exit window defined through the housing in ascanning pattern wider than the exit window such that the single laserbeam is directed through the exit window in a projection pattern, theprojection pattern being smaller than and within the scanning pattern; afirst light detector positioned about a periphery of the exit windowsuch that as the movable mirror apparatus scans the single laser beam inthe scanning pattern, at a point in the scanning pattern where thesingle laser beam is scanned across an interior of the housing and notthrough the exit window, the single laser beam impinges upon the firstlight detector; and a second light detector positioned about theperiphery of the exit window and on a different side thereof than thefirst light detector, the second light detector being located such thatas the movable mirror apparatus scans the single laser beam in thescanning pattern, at a point in the scanning pattern where the singlelaser beam is scanned across the interior of the housing and not throughthe exit window, the single laser beam impinges upon the second lightdetector.
 19. The optical module of claim 18, wherein the first andsecond light detectors are located on opposite sides of the periphery ofthe exit window, outside of the projection pattern but inside thescanning pattern.
 20. The optical module of claim 18, wherein the firstand second light detectors are located on opposite sides of a first axisthat bisects the exit window.
 21. The optical module of claim 20,wherein the first and second light detectors are also located onopposite sides of a second axis perpendicular to the first axis.
 22. Amethod of operating a laser scanning projector contained within ahousing, the method comprising: generating a single laser beam; scanningthe single laser beam across an exit window defined through the housingin a scanning pattern wider than the exit window such that the singlelaser beam is directed through the exit window in a projection pattern,the projection pattern being smaller than and within the scanningpattern, the scanning of the single laser beam being accomplished viadeflection of a movable mirror apparatus; at points in the scanningpattern where the single laser beam is scanned across an interior of thehousing and not through the exit window, detecting the single laser beamas it impinges upon a first light detector positioned about a peripheryof the exit window; adjusting the scanning of the single laser beambased upon the impinging of the single laser beam upon the first lightdetector; wherein adjusting the scanning of the single laser beam basedupon the impinging of the single laser beam upon the first lightdetector comprises: in response to the impinging of the single laserbeam upon the first light detector, digitizing a mirror position signalsupplied by a mirror position sensor associated with the movable mirrorapparatus, and adjusting movement of the movable mirror apparatus basedupon the digitized mirror position signal; and calibrating the mirrorposition sensor based upon the digitized mirror position signal.
 23. Themethod of claim 22, further comprising correcting the digitized mirrorposition signal based upon the calibration of the mirror positionsensor.
 24. The method of claim 22, further comprising ceasinggeneration of the single laser beam based upon lack of impinging of thesingle laser beam upon the first light detector.
 25. The method of claim22, further comprising ceasing generation of the single laser beam basedupon lack of impinging of the single laser beam upon the first lightdetector at an expected time.
 26. The method of claim 22, furthercomprising adjusting the generation of the single laser beam based uponoutput from the first light detector.
 27. A method of operating a laserscanning projector contained within a housing, the method comprising:generating a single laser beam; scanning the single laser beam across anexit window defined through the housing in a scanning pattern wider thanthe exit window such that the single laser beam is directed through theexit window in a projection pattern, the projection pattern beingsmaller than and within the scanning pattern, the scanning of the singlelaser beam being accomplished via deflection of a movable mirrorapparatus; at points in the scanning pattern where the single laser beamis scanned across an interior of the housing and not through the exitwindow, detecting the single laser beam as it impinges upon a firstlight detector positioned about a periphery of the exit window;adjusting the scanning of the single laser beam based upon the impingingof the single laser beam upon the first light detector; whereinadjusting the scanning of the single laser beam based upon the impingingof the single laser beam upon the first light detector comprises: inresponse to the impinging of the single laser beam upon the first lightdetector, digitizing a mirror position signal supplied by a mirrorposition sensor associated with the movable mirror apparatus, andadjusting movement of the movable mirror apparatus based upon thedigitized mirror position signal; determining an instantaneous gain inoutput of the mirror position sensor as a ratio of a calibration valueto the digitized mirror position signal; and scaling the digitizedmirror position signal to account for the instantaneous gain in theoutput of the mirror position sensor prior to adjusting movement of themovable mirror apparatus based upon the digitized mirror positionsignal.
 28. The method of claim 27, further comprising ceasinggeneration of the single laser beam based upon lack of impinging of thesingle laser beam upon the first light detector.
 29. The method of claim27, further comprising ceasing generation of the single laser beam basedupon lack of impinging of the single laser beam upon the first lightdetector at an expected time.